Power unit for a vehicle, and vehicle including same

ABSTRACT

A power unit includes a transmission adapted to transmit rotational power from a crankshaft to a drive axle, with speed change. The transmission has an input shaft rotatably supported on a crankcase. A hydraulic clutch, configured to be selectively engaged and disengaged, is disposed coaxially with the input shaft. A clutch cover covering the hydraulic clutch is connected to the crankcase. A clutch actuator, which controls switching between engagement and disengagement of the hydraulic clutch, and an actuator cover, which covers the clutch actuator in a liquid-tight manner, are mounted to an outer surface of the clutch cover. An oil temperature sensor is mounted to the actuator cover, and is provided for detecting a temperature of oil in an oil chamber, formed between the clutch cover and the actuator cover.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present invention claims priority under 35 USC 119 based on Japanesepatent application No. 2014-193835, filed on Sep. 24, 2014. The entiresubject matter of this priority document, including specification claimsand drawings, is incorporated by reference herein.

BACKGROUND OF THE INVENTION

Technical Field of the Invention

The present invention relates to a power unit in which a transmissionadapted to transmit rotational power from a crankshaft, rotatablysupported by a crankcase, to a drive axle of a vehicle. The power unitis accommodated in, and has an input shaft rotatably supported on thecrankcase. A hydraulic clutch, configured to be selectively engaged anddisengaged to effect and interrupt transmission of power between thecrankshaft and the input shaft, is disposed coaxially with the inputshaft.

Background Art

A power unit wherein an oil temperature sensor for control of the oilpressure in a hydraulic clutch is mounted to an oil supply passageextending from an oil pump to a clutch actuator has already been known,as disclosed in Patent Document 1.

[Patent Document 1] Japanese Patent Laid-Open No. 2010-223395

In a power unit wherein switching between the engagement anddisengagement of a hydraulic clutch is controlled by an operation of aclutch actuator, the oil temperature has a great influence on thecontrol properties.

In a configuration wherein an oil temperature sensor is attached to anoil supply passage extending from an oil pump to a clutch actuator, suchas the one disclosed in Patent Document 1, the oil temperature at thehydraulic clutch is predicted on the basis of the temperature of the oilflowing through the oil supply passage.

In order to enhance the accuracy of control of the hydraulic clutchthrough appropriately coping with variations in oil temperature,therefore, it is desirable to detect the oil temperature at a part ofthe power unit as close to the hydraulic clutch as possible.

SUMMARY OF THE INVENTION

The present invention has been made in consideration of theabove-mentioned circumstances. Accordingly, it is an object of thepresent invention to provide a power unit which makes it possible toaccurately control a hydraulic clutch through appropriately coping withtemperature variations.

Means for Solving the Problem

In order to achieve the above object, according to the presentinvention, there is provided a power unit including a transmissionadapted to transmit rotational power from a crankshaft rotatablysupported by a crankcase, with speed change. The transmission has aninput shaft rotatably supported on the crankcase. The transmission isaccommodated in the crankcase. The power unit further includes ahydraulic clutch engaged and disengaged to effect and interrupttransmission of power between the crankshaft and the input shaft. Thehydraulic clutch is disposed coaxially with the input shaft. In thepower unit, a clutch cover covering the hydraulic clutch is connected tothe crankcase. A clutch actuator which is adapted to control switchingof engaging and disengaging operations of the hydraulic clutch and anactuator cover which covers the clutch actuator in a liquid-tight mannerbetween the clutch cover and itself are mounted to an outer surface ofthe clutch cover. An oil temperature sensor adapted to detect thetemperature of oil accumulated in an oil chamber formed between theclutch cover and the actuator cover is mounted to the actuator cover.

The power unit is characterized secondly, in addition to the firstcharacteristic, in that a hydraulic clutch relief oil passage throughwhich the oil from the hydraulic clutch is discharged into the oilchamber is provided in an actuator body which constitutes part of theclutch actuator and is mounted to the clutch cover.

The power unit is characterized thirdly, in addition to the secondcharacteristic, in that a clutch actuator relief oil passage throughwhich the oil discharged from the clutch actuator is discharged into theoil chamber is provided in the actuator body.

The power unit is characterized fourthly, in addition to the second orthird characteristic, in that an exit of the hydraulic clutch relief oilpassage is provided at a lower portion of the actuator body. The oiltemperature sensor is mounted to the actuator cover, with its detectingportion disposed in a vicinity of the exit of the hydraulic clutchrelief oil passage.

The power unit is characterized fifthly, in addition to any one of thefirst to fourth characteristics, in that an oil return passage throughwhich the oil in the oil chamber is returned to the crankcase side isprovided in an upper portion of the clutch cover so as to be disposedabove the clutch actuator.

Note that the first main shaft 69 and the second main shaft 70 in theembodiment described later correspond to the input shaft in the powerunit of the present invention, the first hydraulic clutch 63 and thesecond hydraulic clutch 64 in the embodiment correspond to the hydraulicclutch in the power unit according to the present invention, and thesecond actuator body 200 in the embodiment corresponds to the actuatorbody in the power unit according to the present invention.

Effects of the Invention

According to the aforementioned first characteristic of the power unitof the present invention, the clutch actuator and the actuator coverwhich covers the actuator body in a liquid-tight manner between theclutch cover and itself are mounted to an outer surface of the clutchcover. In addition, the temperature of the oil accumulated in the oilchamber formed between the clutch cover and the actuator cover isdetected by the oil temperature sensor mounted to the actuator cover.With the clutch actuator and the oil temperature sensor closely disposedin the vicinity of the hydraulic clutch, therefore, it is possible todetect an oil temperature closer to the oil temperature inside thehydraulic clutch. Consequently, the accuracy of control of the hydraulicclutch can be enhanced.

According to the aforementioned second characteristic of the power unitaccording to the present invention, the oil discharged from thehydraulic clutch flows into the oil chamber between the clutch cover andthe actuator cover, and the oil temperature inside the oil chamber isdetected by the oil temperature sensor. Therefore, it is possible todetect an oil temperature closer to the oil temperature inside thehydraulic clutch, and thereby to enhance more the accuracy of control ofthe hydraulic clutch.

According to the aforementioned third characteristic of the power unitaccording to the present invention, the oil discharged from the clutchactuator flows into the oil chamber. Therefore, the oil on the side ofbeing supplied to the hydraulic clutch and the oil on the side of beingdischarged from the hydraulic clutch are mixed with each other in theoil chamber, and the temperature of the thus mixed oil is detected bythe oil temperature sensor. As a result, it is possible to accuratelydetect the temperature of the oil inside the hydraulic clutch, andthereby to further enhance the accuracy of control of the hydraulicclutch.

According to the aforementioned fourth characteristic of the power unitaccording to the present invention, the exit of the hydraulic clutchrelief oil passage for guiding the oil discharged from the hydraulicclutch is provided at a lower portion of the actuator body, and thedetecting portion of the oil temperature sensor is disposed in thevicinity of the exit. Therefore, the temperature of the oil dischargedfrom the hydraulic clutch immediately after starting can be swiftlydetected by the oil temperature sensor, whereby the controllability ofthe hydraulic clutch at the time of warm-up can be enhanced.

According to the aforementioned fifth characteristic of the power unitaccording to the present invention, the oil return passage through whichthe oil in the oil chamber is returned to the crankcase side is providedat an upper portion of the clutch cover so as to be located above theclutch actuator. Therefore, the clutch actuator is located below the oilsurface inside the oil chamber, and can therefore be lagged with thesurrounding oil. Consequently, it is possible to reduce variations inthe temperature of the clutch actuator, and thereby to enhance thehydraulic clutch control performance.

For a more complete understanding of the present invention, the readeris referred to the following detailed description section, which shouldbe read in conjunction with the accompanying drawings. Throughout thefollowing detailed description and in the drawings, like numbers referto like parts.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view of an off-road four-wheel-drive vehicle.

FIG. 2 is a sectional view taken along line 2-2 of FIG. 1.

FIG. 3 is a sectional view taken along line 3-3 of FIG. 2.

FIG. 4 is a back elevation of a power unit as viewed from the rear sideof the vehicle.

FIG. 5 is a perspective view of the power unit as viewed obliquely froma left rear side.

FIG. 6 is a developed sectional view of a power transmission system ofthe power unit.

FIG. 7 is a diagram showing an oil distribution system.

FIG. 8 is a diagram showing the configuration of a clutch actuator.

FIG. 9 is a view of a clutch cover and an actuator cover as viewed fromthe same direction as in FIG. 4.

FIG. 10 is a sectional view taken along line 10-10 of FIG. 9.

FIG. 11 is a view of the clutch cover as viewed in the arrow directionof line 11-11 of FIG. 10.

FIG. 12 is a view of a first partition plate as viewed in the arrowdirection of line 12-12 of FIG. 10.

FIG. 13 is a view of a first actuator body as viewed in the arrowdirection of line 13-13 of FIG. 10.

FIG. 14 is a view of a second partition plate as viewed in the arrowdirection of line 14-14 of FIG. 10.

FIG. 15 is a view of a second actuator body as viewed in the arrowdirection of line 15-15 of FIG. 10.

FIG. 16 is a plan view showing relative disposition of the power unitand a rear differential mechanism.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

An embodiment of the present invention will now be described, withreference to the drawings. Throughout this description, relative termslike “upper”, “lower”, “above”, “below”, “front”, “back”, and the likeare used in reference to a vantage point of an operator of an off-roadfour-wheel-drive vehicle, seated on the driver's seat and facingforward. It should be understood that these terms are used for purposesof illustration, and are not intended to limit the invention.

First, in FIGS. 1 to 3, a pair of left and right front wheels WF aresuspended from a front portion of a body frame F of the off-roadfour-wheel-drive vehicle as a four-wheeled vehicle, and a pair of leftand right rear wheels WR are suspended from a rear portion of the bodyframe F.

The body frame F includes: a pair of left and right lower frames 11extending in the longitudinal vehicle direction; a pair of left andright center riser frames 12 rising upward from intermediate parts inthe longitudinal vehicle direction of the lower frames 11; a pair ofleft and right front side frames 13 extending forward from upper ends ofthe center riser frames 12, then extending forwardly downward andconnected to front portions of the lower frames 11; a pair of left andright rear side frames 14 extending rearward from the upper ends of thecenter riser frames 12, then extending downward and connected to rearportions of the lower frames 11; a center cross member 15interconnecting upper end portions of the pair of left and right centerriser frames 12; a front cross member 16 interconnecting intermediatebent portions of the pair of left and right front side frames 13; anupper rear cross member 17 interconnecting intermediate bent portions ofthe pair of left and right rear side frames 14; and a lower rear crossmember 18 interconnecting lower portions of the pair of left and rightrear side frames 14.

The pair of left and right center riser frames 12 and the pair of leftand right front side frames 13 constitute an outer frame block of afront riding space FS for a driver and a front passenger. A front floor19 disposed forwardly of the center riser frames 12 so as to allow thedriver and the front passenger to ride thereon is supported by a frontportion of the body frame F. In addition, the pair of left and rightcenter riser frames 12 and the pair of left and right rear side frames14 constitute an outer frame block of a rear riding space RS for rearpassengers. A rear floor 20 disposed rearwardly of the center riserframes 12 so as to permit the rear passengers to ride thereon issupported by a rear portion of the body frame F.

In the front riding space FS for the driver and the front passenger, adriver's seat 21 disposed rearwardly of a steering wheel 25 for steeringthe pair of left and right front wheels WF and a first front passenger'sseat 22 disposed on one side in the left-right direction (in thisembodiment, on the right side) of the driver's seat 21 are arrangedseparately from each other in the vehicle width direction.

The driver's seat 21 and the first front passenger's seat 22 includeseat portions 21 a and 22 a and backrest portions 21 b and 22 b risingupward from rear portions of the seat portions 21 a and 22 a,respectively. Besides, a second front passenger's seat 23 is providedbetween the driver's seat 21 and the first front passenger's seat 22.The second front passenger's seat 23 includes a backrest portion 23 bwhich is displaced forward as compared with the backrest portions 21 band 22 b of the driver's seat 21 and the first front passenger's seat22, and a seat portion 23 a which is disposed between the seat portions21 a and 22 a of the driver's seat 21 and the first front passenger'sseat 22. On the other hand, in the rear riding space RS for the rearpassengers, a pair of left and right rear passenger's seats 24 areprovided.

Meanwhile, a front portion of the body frame F is covered with a frontcover 27. In addition, front side covers 28 covering a rear-side lowerportion of the front riding space FS from lateral sides, a pair of leftand right center side covers 29 covering a front-side lower portion ofthe rear riding space RS from lateral sides, and a pair of left andright rear side covers 30 covering a rear-side lower portion of the rearriding space RS from lateral sides, are attached to the body frame F.Besides, on left and right rear portions of the front cover 27, a pairof left and right front doors 33 for opening and closing front entrances31 defined between the front side covers 28 and the front cover 27 areturnably supported by pairs of upper and lower hinge portions 35,respectively. Further, on front portions of the rear side covers 30, apair of left and right rear doors 34 for opening and closing rearentrances 32 defined between the center side covers 29 and the rear sidecovers 30 are turnably supported by pairs of upper and lower hingeportions 36, respectively.

A two-cylinder internal combustion engine E for producing motive powerfor rotationally driving the pair of left and right front wheels WF andthe pair of left and right rear wheels WR is mounted on the body frameF, in the state of being disposed substantially in the center in thelongitudinal vehicle direction in plan view. An engine main body 38 ofthe internal combustion engine E is vertically mounted on the body frameF, with the axis of a crankshaft 39 disposed along the longitudinalvehicle direction and with a cylinder axis C tilted toward the firstfront passenger's seat 22 side in the vehicle width direction. Theengine main body 38 is disposed under the driver's seat 21 and the firstfront passenger's seat 22 in the center in the vehicle width direction.

Paying attention to FIGS. 2 and 3, an intake system 40 of the internalcombustion engine E includes: throttle bodies 41 connected to a cylinderhead 54 of the engine main body 38 on a cylinder basis; an air cleaner42 disposed between the driver's seat 21 and the first front passenger'sseat 22 in plan view; a pair of connecting tubes 43 each interconnectinga throttle body 41 and the air cleaner 42; and a single intake duct 44for guiding air into the air cleaner 42.

Meanwhile, the backrest portion 23 b of the second front passenger'sseat 23 provided between the driver's seat 21 and the first frontpassenger's seat 22 is in integral connection with the backrest portions21 b and 22 b of the driver's seat 21 and the first front passenger'sseat 22, and is displaced forward as compared with the backrest portions21 b and 22 b. As a result, the backrest portions 21 b, 22 b and 23 b ofthe driver's seat 21, the first front passenger's seat 22 and the secondfront passenger's seat 23 form a recess 45 which is recessed to thefront side in plan view. The air cleaner 42 is disposed in the recess45.

A downstream end of the intake duct 44 extending to a right side portionof the vehicle body on the rear side of the seat portion 22 a of thefirst front passenger's seat 22 is connected to a right side portion ofthe air cleaner 42. Resonators 46, 47 and 48 are connected respectivelyto a plurality of parts of the intake duct 44.

A pair of exhaust pipes 50 connected to lower side walls of the cylinderhead 54 of the engine main body 38 are connected to an exhaust muffler51, which is so disposed as to extend in the vehicle width directionalong a rear edge of the body frame F and is supported by the body frameF.

Referring to FIGS. 4 and 5 as well, the engine main body 38 includes: acrankcase 52 on which the crankshaft 39 extending in the longitudinalvehicle direction is rotatably supported; a cylinder block 53 connectedto the crankcase 52, with the cylinder axis C inclined; and the cylinderhead 54 connected to an upper portion of the cylinder block 53.

Besides, a spacer plate 55 constituting a part of the engine main body38 is connected to a back surface of the crankcase 52. A clutch cover 56and an oil storage tank 57 are connected to the crankcase 52 through thespacer plate 55, in such a manner as to project rearward from thecrankcase 52. In addition, a subtransmission case 58 is connected to theopposite side of the axis of the crankshaft 39 from the oil storage tank57, namely, to the front surface side of the crankcase 52. An oil pan 59is connected to a lower portion of the crankcase 52.

Referring to FIG. 6 as well, a transmission system T constituting apower unit P together with the internal combustion engine E is providedbetween the crankshaft 39 of the internal combustion engine E and anintermediate portion of the motive power transmission system between thefront wheels WF and the rear wheels WR (both of which are drivingwheels). The transmission system T includes a transmission 60, and firstand second hydraulic clutches 63 and 64 which are interposed between thetransmission 60 and the crankshaft 39.

An output from the power unit P is transmitted to the left and rightfront wheels WF through a front-wheel propeller shaft 65 (see FIG. 3)extending in the longitudinal vehicle direction, and is transmitted tothe left and right rear wheels WR through a rear-wheel propeller shaft66 (see FIG. 3) extending in the longitudinal vehicle direction. Thefront-wheel propeller shaft 65 and the rear-wheel propeller shaft 66 aredisposed to pass on the right side of the crankcase 52.

The transmission 60 includes a main transmission 61 accommodated in thecrankcase 52, and a subtransmission 62 housed in the subtransmissioncase 58. The subtransmission case 58 includes a first case member 67connected to a front surface of the crankcase 52, and a second casemember 68 which is so disposed that the first case member 67 is locatedbetween itself and the crankcase 52.

The main transmission 61 includes: a first main shaft 69 and a secondmain shaft 70 which are input shafts for input from the crankshaft 39 tothe transmission 60; a counter shaft 71; first, third and fifth speedgear trains G1, G3 and G5 provided between the first main shaft 69 andthe counter shaft 71 so that they can be selectively established; andsecond, fourth and sixth speed gear trains G2, G4 and G6 providedbetween the second main shaft 70 and the counter shaft 71 so that theycan be selectively established.

The first and second main shafts 69 and 70 are relatively rotatablysupported on the crankcase 52 so that the second main shaft 70 coaxiallysurrounds a part of the first main shaft 69. The first and second mainshafts 69 and 70 are disposed on the right side of the crankshaft 39,with their axis parallel to the crankshaft 39. In addition, the countershaft 71 is rotatably supported on the crankcase 52, with its axisparallel to the first and second main shafts 69 and 70.

The selective establishment of the first to sixth speed gear trains G1to G6 are changed over by operating a shift electric motor 72. Asclearly shown in FIG. 4, the shift electric motor 72 is attached to aside projection 55 a provided on the spacer plate 55 so as to project tothe right side from the clutch cover 56.

A power transmission tubular shaft 73 coaxially surrounding the firstmain shaft 69 is supported on the first main shaft 69 at a position onthe axially rear side of and adjacently to the second main shaft 70, insuch a manner as to be rotatable but not movable in the axial directionin relation to the first main shaft 69. The first hydraulic clutch 63 isprovided on the first main shaft 69 so as to be engaged and disengagedto effect and interrupt power transmission between the powertransmission tubular shaft 73 and the first main shaft 69. The secondhydraulic clutch 64 is provided on the power transmission tubular shaft73 and the second main shaft 70 so as to be engaged and disengaged toeffect and interrupt power transmission between the power transmissiontubular shaft 73 and the second main shaft 70.

Rotational power from the crankshaft 39 is transmitted to the powertransmission tubular shaft 73 through a primary speed reduction gear 74and a damper spring 75. The primary speed reduction gear 74 includes aprimary driving gear 76 rotated together with the crankshaft 39, and aprimary driven gear 77 disposed coaxially with the first and second mainshafts 69 and 70 so as to mesh with the primary driving gear 76. Theprimary driven gear 77 is connected to the power transmission tubularshaft 73 through the damper spring 75.

The first hydraulic clutch 63, having a first hydraulic chamber 78, isdisposed on the outer side in the axial direction than the primary speedreduction gear 74. When no hydraulic pressure is exerted on the firsthydraulic chamber 78, the first hydraulic clutch 63 is in a disengagedstate (clutch off state) in which power transmission is interrupted.When a hydraulic pressure is exerted on the first hydraulic chamber 78,the first hydraulic clutch 63 is in an engaged state (clutch on state)in which rotational power transmitted from the crankshaft 39 through theprimary speed reduction gear 74, the damper spring 75 and the powertransmission tubular shaft 73 is transmitted to the first main shaft 69.

The second hydraulic clutch 64, having a second hydraulic chamber 79, isdisposed on the inner side in the axial direction than the firsthydraulic clutch 63 so that the primary speed reduction gear 74 islocated between the second hydraulic clutch 64 and the first hydraulicclutch 63. When no hydraulic pressure is exerted on the second hydraulicchamber 79, the second hydraulic clutch 64 is in a disengaged state(clutch off state) in which power transmission is interrupted. When ahydraulic pressure is exerted on the second hydraulic chamber 79, thesecond hydraulic clutch 64 is in an engaged state (clutch on state) inwhich rotational power transmitted from the crankshaft 39 through theprimary speed reduction gear 74, the damper spring 75 and the powertransmission tubular shaft 73 is transmitted to the second main shaft70.

The first main shaft 69 is provided therein with mutually parallel firstand second axial oil passages 80 and 81 which are closed at inner endsthereof and extend in the axial direction. The first axial oil passage80 communicates with the first hydraulic chamber 78, whereas the secondaxial oil passage 81 communicates with the second hydraulic chamber 79.In addition, a first oil passage 82 communicating with the first axialoil passage 80 and a second oil passage 83 communicating with the secondaxial oil passage 81 are formed in the clutch cover 56.

The subtransmission 62 includes a speed change driving shaft 85, an idleshaft 86 and a driving force output shaft 87. The speed change drivingshaft 85 extends in the longitudinal vehicle direction, coaxially withthe counter shaft 71 of the main transmission 61, and is rotatablysupported on first and second case members 67 and 68 of thesubtransmission case 58. Besides, a rear end portion of the speed changedriving shaft 85 penetrates the first case member 67 in a rotatablemanner, to protrude to the crankcase 52 side. A front end portion of thecounter shaft 71 penetrating the crankcase 52 in a rotatable manner iscoaxially coupled to the rear end portion of the speed change drivingshaft 85 through a damper mechanism 88. In other words, rotational powerof the counter shaft 71 is transmitted through the damper mechanism 88to the speed change driving shaft 85.

In the subtransmission case 58, a high-speed driving gear 89, alow-speed driving gear 90 and a reverse driving gear 91 are supported onthe speed change driving shaft 85 in a relatively rotatable manner,while being located in this order from the front side. Between thesecond case member 68 and the high-speed driving gear 89, a high-speedswitching shifter 92 not rotatable relative to the speed change drivingshaft 85 is slidably supported on the speed change driving shaft 85 soas to be switchable between a position for engagement with thehigh-speed driving gear 89 and a neutral position for disengagement fromthe high-speed driving gear 89. Between the low-speed driving gear 90and the reverse driving gear 91, a forward-reverse switching shifter 93not rotatable relative to the speed change driving shaft 85 is slidablysupported on the speed change driving shaft 85 so as to be switchedamong a position for engagement with the low-speed driving gear 90, aposition for engagement with the reverse driving gear 91, and a neutralposition for engagement with neither of the low-speed driving gear 90and the reverse driving gear 91.

The idle shaft 86 is formed in a hollow cylindrical shape surrounding asupport shaft 94 which is rotatably supported on the first and secondcase members 67 and 68 of the subtransmission case 58. The idle shaft 86is supported on the support shaft 94 in a relatively rotatable manner.Inside the subtransmission case 58, the idle shaft 86 is integrallyprovided at a front portion and a rear portion thereof with asmall-diameter idle gear 95 and a large-diameter idle gear 96. Thelarge-diameter idle gear 96 is meshed with the reverse driving gear 91.

Besides, a hollow cylindrical boss 97 is fixed to the driving forceoutput shaft 87 in the manner of being disposed in a positioncorresponding to the small-diameter idle gear 95 and to the high-speeddriving gear 89 and the low-speed driving gear 90 of the speed changedriving shaft 85. A small-diameter driven gear 99 is connected to theboss 97 through a damper spring 98, and a large-diameter driven gear 101is connected to the boss 97 through a damper spring 100. The high-speeddriving gear 89 is meshed with the small-diameter driven gear 99,whereas the low-speed driving gear 90 and the small-diameter idle gear95 are meshed with the large-diameter driven gear 101.

A first shift fork 102 rotatably embracing the high-speed switchingshifter 92 and a second shift fork 103 rotatably embracing theforward-reverse switching shifter 93 are slidably supported on a shiftfork shaft 104, which is supported on the first and second case members67 and 68 of the subtransmission case 58 while having an axis parallelto the speed change driving shaft 85. In addition, a shift drum 105having an axis parallel to the shift fork shaft 104 is turnablysupported on the first and second case members 67 and 68. The first andsecond shift forks 102 and 103 are engaged with guide grooves 106 and107 provided in an outer circumferential surface of the shift drum 105.

As the shift drum 105 is turned, the first and second shift forks 102and 103 are moved along the shift fork shaft 104, whereby the high-speeddriving gear 89, the low-speed driving gear 90 and the reverse drivinggear 91 are selectively connected to the speed change driving shaft 85in a relatively non-rotatable manner.

The subtransmission case 58 is formed with an output shaft supportportion 110 projecting more to the right side than the crankcase 52. Thedriving force output shaft 87 offset in the axial direction in relationto the first and second main shafts 69 and 70 is rotatably supported onthe output shaft support portion 110. A rear portion of the drivingforce output shaft 87 penetrates the output shaft support portion 110 ina rotatable manner, to protrude to the rear side, in a condition inwhich a ball bearing 111 and an oil seal 112 disposed on the outer sideof the ball bearing 111 are interposed between the rear portion and theoutput shaft support portion 110 on the first case member 67 side. Therear-wheel propeller shaft 66, extending on the right side of thecrankcase 52, is connected to a rear end portion of the output shaftsupport portion 110. In addition, a front portion of the driving forceoutput shaft 87 penetrates the output shaft support portion 110 in arotatable manner, to protrude to the front side, in a condition in whicha ball bearing 113 and an oil seal 114 disposed on the outer side of theball bearing 113 are interposed between the front portion and the outputshaft support portion 110 on the second case member 68 side. Thefront-wheel propeller shaft 65 extending on the right side of thecrankcase 52 is connected to a front end portion of the output shaftsupport portion 110.

Between the first case member 67 and the second case member 68, there isformed a breather chamber 115 having at least a part thereof disposedover a protruding part of a rear end portion of the driving force outputshaft 87 which protrudes from the output shaft support portion 110.

The first case member 67 is integrally formed with a rearwardlyprotruding portion 116, at a part corresponding to an upper part of theoutput shaft support portion 110. The breather chamber 115 is formedbetween the protruding portion 116 and an upper portion of the secondcase member 68 at a position corresponding to the output shaft supportportion 110. Besides, the second case member 68 of the subtransmissioncase 58 is integrally formed with a tubular portion 68 a extendingforward along the axial direction of the driving force output shaft 87so as to cover the driving force output shaft 87 on a lateral side ofthe latter.

In FIG. 7, the engagement and disengagement of the first and secondhydraulic clutches 63 and 64 are switched by a clutch actuator 125,which is disposed at the clutch cover 56. Meanwhile, an oil sump 126 fortemporary storage of oil is formed between the spacer plate 55 and theoil storage tank 57. An oil from a first oil feeding pump 127 forsucking up the oil from the oil sump 126 is supplied through a first oilfilter 130 to the clutch actuator 125. The clutch actuator 125 operatesso as to switch the action of and release of action of a hydraulicpressure on the first oil passage 82 and the first axial oil passage 80communicating with the first hydraulic chamber 78 of the first hydraulicclutch 63, and the action of and release of action of a hydraulicpressure on the second oil passage 83 and the second axial oil passage81 communicating with the second hydraulic chamber 79 of the secondhydraulic clutch 64. By the operation of the clutch actuator 125, theengagement and disengagement of the first and second hydraulic clutches63 and 64 are switched over. In addition, a surplus oil from the firstoil feeding pump 127 is returned to the oil sump 126 via a first reliefvalve 133.

A second oil feeding pump 128 is connected to the oil sump 126, incommon with the first oil feeding pump 127. The oil from the second oilfeeding pump 128 is supplied through a second oil filter 131 to aplurality of lubricated portions 132 of the power unit P. A surplus oilfrom the second oil feeding pump 128 is returned to the oil sump 126 viaa second relief valve 134.

The discharge capacity of the second oil feeding pump 128 is set to begreater than the discharge capacity of the first oil feeding pump 127.Therefore, the second oil filter 131 is greater than the first oilfilter 130 in size.

In addition, the oil in the above-mentioned oil pan 59 is sucked up by ascavenger pump 129 via a strainer 135. The oil discharged from thescavenger pump 129 is supplied to the oil sump 126 via an oil cooler136.

Paying attention to FIG. 4, the first oil feeding pump 127, the secondoil feeding pump 128 and the scavenger pump 129 are coaxially disposedwhile aligned in parallel to the axis of the crankshaft 39 so as to bedriven by rotational power transmitted from the crankshaft 39, cooperatewith each other in constituting a pump unit 138, and are disposed at theabove-mentioned spacer plate 55.

The oil storage tank 57, having a recess 140 for accommodating a part ofthe clutch cover 56, is disposed adjacently to the clutch cover 56. Onthe opposite side of the crankshaft 39 from the transmission 60, namely,on the opposite side from the clutch cover 56, the first and second oilfilters 130 and 131 are mounted to an outside wall 57 a of the oilstorage tank 57 while being aligned in the vertical direction. Inaddition, the oil cooler 136 is mounted to an outer surface, facingtoward the vehicle rear side, of an upper portion of the oil storagetank 57.

In FIG. 8, the clutch actuator 125 includes: a linear solenoid valve 141as a hydraulic pressure control valve; an oil passage switching valve142 for supplying the oil from the linear solenoid valve 141alternatively and selectively into the first and second hydraulicchambers 78 and 79 of the first and second hydraulic clutches 63 and 64;a switching control valve 143 for switching control of the supply anddischarge of the oil for a switching operation of the oil passageswitching valve 142; a pair of shift solenoid valves 144 and 145 asvalves for control of a switching operation of the switching controlvalve 143; and a discharge control solenoid valve 146 as a valve foropening/closing of a discharge oil passage 147 connected alternativelyand selectively to the first and second hydraulic chambers 78 and 79 ofthe first and second hydraulic clutches 63 and 64.

The linear solenoid valve 141 includes an input port 148, an output port149, and a feedback port 150 communicating with the output port 149. Thelinear solenoid valve 141 controls the oil pressure at the input port148 (where the oil is inputted) to a pressure according to an electriccurrent supplied, and outputs the oil at the controlled pressure via theoutput port 149. An oil supply passage 151 for guiding the oil suppliedfrom the first oil feeding pump 127 through the first oil filter 130 isconnected to the input port 148.

The oil passage switching valve 142 includes: a first output port 152connected to the first oil passage 82 communicating with the firsthydraulic chamber 78 of the first hydraulic clutch 63; a second outputport 153 connected to the second oil passage 83 communicating with thesecond hydraulic chamber 79 of the second hydraulic clutch 64; an inputport 154 communicating with a connection oil passage 160; a pair offirst and second discharge ports 155 and 156 commonly connected to thedischarge oil passage 147; and a spool valve body 159 so disposed thatone end thereof is exposed to a pilot chamber 157 and a spring force ofa return spring 158 acts on the other end thereof. The oil controlled inpressure by the linear solenoid valve 141, or the oil from a firstbranch oil supply passage 151 a having an orifice 167, branched from theoil supply passage 151 and bypassing the linear solenoid valve 141, isguided into the connection oil passage 160.

The switching control valve 143 includes: an output port 161 connectedto the pilot chamber 157 of the oil passage switching valve 142; aninput port 162 connected to a second branch oil supply passage 151 bwhich has an orifice 168 and is branched from the oil supply passage151; a relief port 163; and a spool valve body 166 so disposed that oneend thereof is exposed to a pilot chamber 164 and a spring force of areturn spring 165 acts on the other end thereof. A relief oil passage170 having an orifice 169 is connected to the relief port 163.

The pair of shift solenoid valves 144 and 145 are connected in parallelbetween the oil supply passage 151 and the pilot chamber 164 so as toswitch between a state in which the oil supply passage 151 communicateswith the pilot chamber 164 and a state in which the oil in the pilotchamber 164 is relieved to the exterior.

Besides, the discharge control solenoid valve 146 can switch between astate in which the working oil in the discharge oil passage 147 isrelieved to the exterior via a relief oil passage 172 having an orifice171 and a state in which the working oil in the discharge oil passage147 is guided to a relief oil passage 173 in the manner of beingrelieved to the exterior without constriction.

In accordance with the clutch actuator 125 as above, the following isensured. In a state in which the oil supply passage 151 is made tocommunicate with the pilot chamber 164 of the switching control valve143 by the pair of shift solenoid valves 144 and 145, the spool valvebody 166 of the switching control valve 143 is in such a position as tocause the input port 162 communicating with the second branch oil supplypassage 151 b to communicate with the output port 161, and as to cut offcommunication of the relief port 163 with the output port 161. Inresponse to such an operation of the switching control valve 143, thespool valve body 159 of the oil passage switching valve 142 comes intosuch a position as to cut off communication of the first output port 152with the first discharge port 155 although allowing the first outputport 152 to communicate with the connection oil passage 160, and as tocut off communication of the second output port 153 with the connectionoil passage 160 although allowing the second output port 153 tocommunicate with the second discharge port 156.

When the oil in the pilot chamber 164 of the switching control valve 143is relieved to the exterior by the pair of shift solenoid valves 144 and145, the spool valve body 166 of the switching control valve 143 comesinto such a position as to cut off communication of the input port 162(which communicates with the second branch oil supply passage 151 b)with the output port 161, and as to cause the relief port 163 tocommunicate with the output port 161. In response to such an operationof the switching control valve 143, the spool valve body 159 of the oilpassage switching valve 142 comes into such a position as to cut offcommunication of the second output port 153 with the second dischargeport 156 although allowing the second output port 153 to communicatewith the connection oil passage 160, and as to cut off communication ofthe first output port 152 with the connection oil passage 160 althoughallowing the first output port 152 to communicate with the firstdischarge port 155.

Between the linear solenoid valve 141 and the connection oil passage 160communicating with the input port 154 of the oil passage switching valve142, there is provided a manual switching valve 175. The manualswitching valve 175 enables the oil from the first oil feeding pump 127to be guided to the oil passage switching valve 142 side via the firstbranch oil supply passage 151 a which bypasses the linear solenoid valve141.

The manual switching valve 175 includes: a first input port 176communicating with the output port 149 of the linear solenoid valve 141;a second input port 177 communicating with the first branch oil supplypassage 151 a; an output port 178 communicating with the connection oilpassage 160; a relief port 179 connected to a relief passage 181 havingan orifice 180; a spool valve body 182 movable between such a positionas to cause the first input port 176 to communicate with the output port178 and such a position as to cause the second input port 177 tocommunicate with the output port 178; a spring 183 biasing the spoolvalve body 182 toward such a position as to cause the first input port176 to communicate with the output port 178; and an operating piston 184making contact with the spool valve body 182 from the opposite side fromthe spring 183 while enabling the spool valve body 182 to be pushed tomove, against the biasing force of the spring 183, toward such aposition as to cause the second input port 177 to communicate with theoutput port 178.

The operating piston 184 is screw-engaged with the clutch cover 56 in anadvanceable and retractable manner. The operating piston 184 is formedwith an annular locking recess 185 at the outer periphery of an outerend portion thereof. An engaging plate 186 engageable with the lockingrecess 185 is fastened to the clutch cover 56 by a screw member 187. Ina state in which the engaging plate 186 fastened to the clutch cover 56is engaged with the locking recess 185, the spool valve body 182 is in aretracted position shown in FIG. 8. When the operating piston 184 isadvanced in a screwing-in manner in a state in which the engaging plate186 is unfastened from the clutch cover 56 and is disengaged from thelocking recess 185, the spool valve body 182 is moved into an advancedposition. The advanced position of the spool valve body 182 ismaintained by engagement of the engaging plate 186, which is fastened tothe clutch cover 56, with the outer end of the operating piston 184.

The spool valve body 182 is formed in the outer periphery thereof withan annular recess 188 which communicates with the output port 178 but isinhibited from communication with the second input port 177,irrespectively of the position of the spool valve body 182. The annularrecess 188 is so formed that when the spool valve body 182 is in theretracted position, the second input port 177 is let communicate withthe relief port 179, but when the spool valve body 182 is in theadvanced position, the second input port 177 is inhibited fromcommunication with the first input port 176. In addition, the spoolvalve body 182 is formed in the outer periphery thereof with an annulargroove 189. The annular groove 189 is so formed that when the spoolvalve body 182 is in the advanced position, the annular groove 189communicates with the second input port 177 and the relief port 179, butwhen the spool valve body 182 is in the retracted position, the annulargroove 189 is inhibited from communication with the second input port177 and the relief port 179. Furthermore, the spool valve body 182 isprovided therein with an orifice passage 190 which interconnects theannular recess 188 and the annular groove 189.

The oil supply passage 151 is provided with a first oil pressure sensor191 for detection of an oil supply pressure that is supplied from thefirst oil feeding pump 127 through the first oil filter 130. The firstoil passage 82 is provided with a second oil pressure sensor 192 fordetection of an oil pressure acting on the first hydraulic clutch 63.The second oil passage 83 is provided with a third oil pressure sensor193 for detection of an oil pressure acting on the second hydraulicclutch 64.

Referring to FIGS. 9 to 11 as well, an actuator mounting portion 56 aonto which to mount the clutch actuator 125 is provided to integrallyproject from an outer surface of the clutch cover 56. The manualswitching valve 175 is mounted to the actuator mounting portion 56 a soas to extend in an up-down direction while inclining to be higher on theright side. In addition, the operating piston 184 which can be manuallyoperated in emergency, such as upon a trouble with the clutch actuator125, is screw-engaged with the actuator mounting portion 56 a in anadvanceable and retractable manner, with its outer end portion exposedto a right outer side of the actuator mounting portion 56 a. Theoperating piston 184 is formed at its outer end with a tool lockingrecess 194 into which a tool can be engaged so as to rotate theoperating piston 184 thereby.

In addition to the linear solenoid valve 141, the oil passage switchingvalve 142, the switching control valve 143, the shift solenoid valves144 and 145 and the discharge control solenoid valve 146, the clutchactuator 125 is provided with a first partition plate 197, a firstactuator body 198, a second partition plate 199 and a second actuatorbody 200 for supporting the valves 141 to 146 and for constituting oilpassages extending between the valves 141 to 146. The first actuatorbody 198 is mounted to the actuator mounting portion 56 a via the firstpartition plate 197. The second actuator body 200 overlapping with thefirst actuator body 198 from the opposite side from the clutch cover 56is mounted to the first actuator body 198, with the second partitionplate 199 interposed between the first actuator body 198 and the secondactuator body 200.

Meanwhile, the actuator mounting portion 56 a is provided at an outersurface of the clutch cover 56 while disposed in such a position that atleast part thereof overlaps with the first and second hydraulic clutches63 and 64 as viewed in the direction along the axis CA of the first andsecond main shafts 69 and 70 of the transmission 60. In this embodiment,the actuator mounting portion 56 a is disposed at a position offset tothe left side in the vehicle width direction, with reference to the axisCA of the first and second main shafts 69 and 70.

The first partition plate 197 and the first actuator body 198 aremounted to the actuator mounting portion 56 a, at a position more offsetto the left side in the vehicle width direction than the actuatormounting portion 56 a, with reference to the axis CA of the first andsecond main shafts 69 and 70. As shown in FIG. 12, the first partitionplate 197 is provided in its peripheral portion with six insertion holes201 spaced apart along the circumferential direction. As shown in FIG.13, the first actuator body 198 is provided in its peripheral portionwith six insertion holes 202 spaced apart along the circumferentialdirection, correspondingly to the insertion holes 201. Further, as shownin FIG. 11, the actuator mounting portion 56 a is provided with sixbottomed screw holes 203 corresponding to the insertion holes 201 and202.

In addition, at an intermediate portion in the vertical direction of aright side surface of the first actuator body 198, a boss portion 198 ais integrally provided so as to protrude more to the right side than thefirst partition plate 197. The boss portion 198 a is provided with abottomed screw hole 204, which is opened to the second partition plate199 side.

In FIG. 14, the second partition plate 199 is provided with sixinsertion holes 205 corresponding to the insertion holes 201 and 202 inthe first partition plate 197 and the first actuator body 198, and isprovided also with an insertion hole 206 corresponding to the screw hole204 in the boss portion 198 a of the first actuator body 198.

In FIG. 15, the second actuator body 200 is provided with six insertionholes 207 corresponding to the insertion holes 201, 202, and 205 in thefirst partition plate 197, the first actuator body 198 and the secondpartition plate 199, and is provided also with an insertion hole 208corresponding to the insertion hole 206 in the second partition plate199.

The first partition plate 197, the second partition plate 199 and thesecond actuator body 200 are mounted to the first actuator body 198 by amethod wherein bolts 209 passed through the insertion holes 201, 202,205, and 207 are screw-engaged into the screw holes 203 in the actuatormounting portion 56 a of the clutch cover 56 and are fastened to theactuator mounting portion 56 a, and wherein bolts 210 passed through theinsertion holes 206 and 208 are screw-engaged into the screw holes 204in the first actuator body 198 and are fastened to the latter.

The switching control valve 143 and the shift solenoid valves 144 and145 as some of the linear solenoid valve 141, the oil passage switchingvalve 142, the switching control valve 143, the shift solenoid valves144 and 145 and the discharge control solenoid valve 146, which are theplurality of valves constituting part of the clutch actuator 125, aredisposed in the first actuator body 198.

The first actuator body 198 is provided therein with first and secondvalve mounting holes 211 and 212 which extend horizontally along a planeorthogonally intersecting the axis of the first and second hydraulicclutches 63 and 64, namely, the axis CA of the first and second mainshafts 69 and 70 and opening in a right side surface of the firstactuator body 198, in such a manner that the shift solenoid valves 144and 145 are mounted in the valve mounting holes 211 and 212. Inaddition, the first actuator body 198 is provided therein with a thirdvalve mounting hole 213 disposed in parallel to the valve mounting holes211 and 212 and opening in a left side surface of the first actuatorbody 198, in such a manner that the switching control valve 143 ismounted in the third valve mounting hole 213.

Specifically, the switching control valve 143 and the shift solenoidvalves 144 and 145 are disposed in the first actuator body 198 in avertically aligned state, with their axes of operation set horizontalalong a plane orthogonally intersecting the axis CA of the first andsecond main shafts 69 and 70.

Besides, the linear solenoid valve 141, the oil passage switching valve142 and the discharge control solenoid valve 146, which are the valvesremaining after excluding the switching control valve 143 and the shiftsolenoid valves 144 and 145 from the linear solenoid valve 141, the oilpassage switching valve 142, the switching control valve 143, the shiftsolenoid valves 144 and 145 and the discharge control solenoid valve146, are disposed in the second actuator body 200.

The second actuator body 200 is provided therein with a fourth valvemounting hole 214 and a fifth valve mounting hole 215 at positionsspaced apart along the vertical direction. The fourth valve mountinghole 214 extends horizontally along a plane orthogonally intersectingthe axis CA of the first and second main shafts 69 and 70, and has bothends thereof opening in both left and right side surfaces of the secondactuator body 200. The fifth valve mounting hole 215 extendshorizontally along a plane orthogonally intersecting the axis CA of thefirst and second main shafts 69 and 70, and is opening in a right sidesurface of the second actuator body 200. The linear solenoid valve 141is mounted in the fourth valve mounting hole 214, and the oil passageswitching valve 142 is mounted in the fifth valve mounting hole 215.

Besides, the second actuator body 200 is provided in its lower portionwith a sixth valve mounting hole 216 located under the fifth valvemounting hole 215. The sixth valve mounting hole 216 extends in parallelto the axis CA of the first and second main shafts 69 and 70, and opensto the opposite side from the clutch cover 56. The discharge controlsolenoid valve 146 is mounted in the sixth valve mounting hole 216.

Meanwhile, part of the oil passages extending between the linearsolenoid valve 141, the oil passage switching valve 142, the switchingcontrol valve 143, the shift solenoid valves 144 and 145 and thedischarge control solenoid valve 146 are composed of pluralities ofgrooves, through-holes and bottomed holes provided in the first andsecond actuator bodies 198 and 200 and through-holes provided in thefirst and second partition plates 197 and 199.

The actuator mounting portion 56 a of the clutch cover 56 is provided inits peripheral portion with a plurality of screw holes 218 spaced fromone another. An actuator cover 220 for covering the clutch actuator 125in a liquid-tight manner between itself and the actuator mountingportion 56 a is mounted to the actuator mounting portion 56 a by aplurality of bolts 219 screw-engaged into the screw holes 218. An oilchamber 221 is formed between the actuator mounting portion 56 a of theclutch cover 56 and the actuator cover 220.

Under the first and second actuator bodies 198 and 200, specificallyunder the actuator cover 220 covering the first and second actuatorbodies 198 and 200 in this embodiment, the second and third oil pressuresensors 192 and 193 for individual detection of oil pressures acting onthe first and second hydraulic clutches 63 and 64 are mounted to a lowerouter surface of the clutch cover 56, so as to be aligned with eachother along the vehicle width direction. The first oil pressure sensor191 for detection of the pressure of the oil supplied from the first oilfeeding pump 127 is mounted to the lower outer surface of the clutchcover 56, at such a position that the second oil pressure sensor 192 isinterposed between the first oil pressure sensor 191 and the third oilpressure sensor 193.

The temperature of the oil accumulated in the oil chamber 221 isdetected by an oil temperature sensor 222. The oil temperature sensor222 is mounted to a left-side lower portion of the actuator cover 220,in such a manner that its detecting portion 222 a is disposed at a lowerportion inside the oil chamber 221 so as to overlap with a left-sidelower portion of the actuator body 200 as viewed in the direction alongthe axis of the first and second main shafts 69 and 70.

Moreover, in order to discharge into the oil chamber 221 the oil comingfrom the first and second hydraulic clutches 63 and 64, a hydraulicclutch relief oil passage 223 (see FIG. 15) communicating with therelief oil passages 172 and 173 is provided so that its exit opens in alower portion of the second actuator body 200. In other words, the oiltemperature sensor 222 is mounted to the actuator cover 220 so that itsdetecting portion 222 a is disposed in the vicinity of the exit of thehydraulic clutch relief oil passage 223.

In addition, for example, three clutch actuator relief oil passages 224(see FIG. 15) for discharging into the oil chamber 221 the oildischarged from the clutch actuator 125 are provided in an upper portionof the second actuator body 200.

Besides, an oil return passage 225 through which the oil in the oilchamber 221 is returned to the crankcase 52 side is provided at an upperportion of the actuator mounting portion 56 a of the clutch cover 56, soas to be disposed above the clutch actuator 125. The first partitionplate 197, the first actuator body 198, the second partition plate 199and the second actuator body 200 are formed with recesses 226, 227, 228,and 229, respectively, disposed between two bolts 209 for mounting upperportions of these members 197 to 200 to the actuator mounting portion 56a so that the oil return passage 225 is exposed at the recesses 226 to229.

In FIG. 16, the crankshaft 39 of the internal combustion engine E in thepower unit P is disposed with its axis oriented in the longitudinalvehicle direction so as to be orthogonal to the axis of a pair ofdriving shafts 230 connected with the pair of left and right rear wheelsWR. In addition, the rear-wheel propeller shaft 66 for transmitting adriving force from the driving force output shaft 87 of the transmission60 disposed on the right side in the left-right direction of thecrankshaft 39 to the side of the left and right rear wheels WR is alsoextended in a direction parallel to the crankshaft 39, namely, in adirection orthogonal to the axis of the driving shafts 230. A rear endportion of the rear-wheel propeller shaft 66 is connected to adifferential mechanism 231 provided between the pair of left and rightdriving shafts 230.

Moreover, the clutch actuator 125 and the actuator cover 220 coveringthe clutch actuator 125 are mounted to the clutch cover 56, which isdisposed to face the differential mechanism 231 from the front side, ata position offset from the center axis of the first and second hydraulicclutches 63 and 64, namely, from the axis CA of the first and secondmain shafts 69 and 70. The clutch actuator 125 and the actuator cover220 are disposed on the other side in the left-right direction, withreference to a shaft retaining portion 231 a which constitutes part ofthe differential mechanism 231 and which retains an end portion, on thedifferential mechanism 231 side, of the propeller shaft 66.Specifically, the transmission 60 is disposed on the right side in theleft-right direction, with respect to the crankshaft 39. On the otherhand, the clutch actuator 125 and the actuator cover 220 are mounted tothe clutch cover 56 at a position offset from the axis CA of the firstand second main shafts 69 and 70, in such a manner as to be disposed onthe left side in the left-right direction, with reference to the shaftretaining portion 231 a.

Operation of this embodiment will now be described below. The first andsecond hydraulic clutches 63 and 64 which are engaged and disengaged toeffect and interrupt power transmission between the crankshaft 39 andthe first and second main shafts 69 and 70 are covered by the clutchcover 56 connected to the crankcase 52. The clutch actuator 125 forcontrolling the switching of the engagement and disengagement of thefirst and second hydraulic clutches 63 and 64 is mounted to the clutchcover 56. The switching control valve 143 and the shift solenoid valves144 and 145, which are some valves of the plurality of valves 141, 142,143, 144, 145 and 146 constituting part of the clutch actuator 125, aredisposed in the first actuator body 198 mounted to an outer surface ofthe clutch cover 56 while disposed in such a position as to at leastpartly overlap with the first and second hydraulic clutches 63 and 64when viewed in the direction along the axis CA of the first and secondmain shafts 69 and 70. The linear solenoid valve 141, the oil passageswitching valve 142 and the discharge control solenoid valve 146, whichare the remaining valves of the plurality of valves 141 to 146, aredisposed in the second actuator body 200 mounted to the clutch cover 56so as to overlap with the first actuator body 198 from the opposite sidefrom the clutch cover 56. Therefore, it is possible to avoid an increasein the size of the clutch cover 56 in the radial direction of the firstand second hydraulic clutches 63 and 64, and it is easy to form oilpassages extending between the valves. In addition, the axes ofoperation of the switching control valve 143 and the shift solenoidvalves 144 and 145 disposed in the first actuator body 198 are along aplane intersecting the axis CA of the first and second main shafts 69and 70. Accordingly, it is possible to reduce the amount of extrusion ofthe clutch actuator 125 from the clutch cover 56 in the direction alongthe axis CA of the first and second main shafts 69 and 70, and toobviate an increase in the size of the vehicle on which the power unit Pis mounted.

The switching control valve 143 and the shift solenoid valves 144 and145 are disposed in the first actuator body 198 so as to be aligned inthe vertical direction, with the axes of operation of the valves 143 to145 set parallel to one another. Therefore, the first actuator body 198can be made smaller in size while ensuring stable support of theswitching control valve 143 and the shift solenoid valves 144 and 145.

The second and third oil pressure sensors 192 and 193 for individualdetection of the oil pressures acting on the first and second hydraulicclutches 63 and 64 and the first oil pressure sensor 191 for detectingthe pressure of the oil supplied from the first oil feeding pump 127 tothe clutch actuator 125 side are provided on an outer surface of theclutch cover 56, under the first and second actuator bodies 198 and 200,specifically, under the actuator cover 220 covering the clutch actuator125 including the first and second actuator bodies 198 and 200 in thisembodiment. Therefore, the first to third oil pressure sensors 191 to193 can be arranged compactly by utilizing a vacant space around thefirst and second actuator bodies 198 and 200. With detecting portions ofthe first to third oil pressure sensors 191 to 193 set at lowerpositions, upward air release properties can be enhanced. Consequently,it is possible to enhance the accuracy of oil pressure detection, andhence to enhance the accuracy of engagement-disengagement switchingcontrol of the first and second hydraulic clutches 63 and 64.

The switching control valve 143 is disposed between the pair of shiftsolenoid valves 144 and 145 so as to effect switching between an oilpressure exertion state and an oil pressure relief state of the firstand second hydraulic clutches 63 and 64 by operating in response tooperations of the shift solenoid valves 144 and 145. The pair of shiftsolenoid valves 144 and 145 and the switching control valve 143 arearranged on the first actuator body 198. Therefore, by switching betweenthe oil pressure exertion state and the oil pressure relief state of thefirst and second hydraulic clutches 63 and 64 according to theoperations of the pair of shift solenoid valves 144 and 145, it ispossible to enhance the reliability while contriving a reduction in thesize of the shift solenoid valves 144 and 145. The switching controlvalve 143 can be effectively disposed in the space generated between thepair of small-sized shift solenoid valves 144 and 145, whereby theclutch actuator 125 can be made smaller in size.

In addition, the crankcase 52 is disposed with the axis of thecrankshaft 39 oriented in the direction orthogonal to the axis of thepair of driving shafts 230 connected to the pair of left and right rearwheels WR. The transmission 60 disposed on the right side, in theleft-right direction, of the crankshaft 39 is accommodated in thecrankcase 52. The clutch cover 56 is connected to the differentialmechanism 231, provided between the pair of driving shafts 230, in themanner of facing the differential mechanism 231 from the front side inthe longitudinal vehicle direction. The rear-wheel propeller shaft 66disposed in parallel to the crankshaft 39 is connected to thedifferential mechanism 231 so as to transmit the driving force outputtedfrom the transmission 60. The shaft retaining portion 231 a constitutespart of the differential mechanism 231, and retains an end portion, onthe differential mechanism 231 side, of the rear-wheel propeller shaft66. The clutch actuator 125 is mounted to the clutch cover 56 at aposition offset from the center axis CA of the first and secondhydraulic clutches 63 and 64, so as to be disposed on the left side (inthe left-right direction) of the shaft retaining portion 231 a. With thepower unit P and the differential mechanism 231 set close to each other,therefore, the power unit P can be arranged in a position nearer to theleft and right rear wheels WR, whereby a vehicle enhanced in compactnesscan be realized.

The clutch actuator 125 is mounted to an outer surface of the clutchcover 56, and the actuator cover 220 for covering the clutch actuator125 in a liquid-tight manner between itself and the clutch cover 56 ismounted to the clutch cover 56. Besides, the oil temperature sensor 222for detecting the temperature of the oil accumulated in the oil chamber221 formed between the clutch cover 56 and the actuator cover 220 ismounted to the actuator cover 220. Therefore, by closely arranging theclutch actuator 125 and the oil temperature sensor 222 in the vicinityof the first and second hydraulic clutches 63 and 64, it is possible todetect a temperature which is closer to the temperatures inside thefirst and second hydraulic clutches 63 and 64. Accordingly, the accuracyof control of the first and second hydraulic clutches 63 and 64 can beenhanced.

The hydraulic clutch relief oil passage 223 through which the oil fromthe first and second hydraulic clutches 63 and 64 is discharged into theoil chamber 221 is provided in the second actuator body 200 thatconstitutes part of the clutch actuator 125 and is mounted to the clutchcover 56. Therefore, the temperature of the oil discharged from thefirst and second hydraulic clutches 63 and 64 and flowing into the oilchamber 221 is detected by the oil temperature sensor 220. Consequently,it is possible to detect a temperature closer to the temperatures of theoil inside the first and second hydraulic clutches 63 and 64, andthereby to enhance more the accuracy of control of the first and secondhydraulic clutches 63 and 64.

The clutch actuator relief oil passage 224 through which the oildischarged from the clutch actuator 125 is discharged into the oilchamber 221 is provided in the second actuator body 200. Therefore, theoil on the side of being supplied to the first and second hydraulicclutches 63 and 64 and the oil on the side of being discharged from thefirst and second hydraulic clutches 63 and 64 are mixed with each otherin the oil chamber 221, and the temperature of the thus mixed oil isdetected by the oil temperature sensor 222. Accordingly, it is possibleto accurately detect the temperature of the oil in the first and secondhydraulic clutches 63 and 64, and thereby to further enhance theaccuracy of control of the first and second hydraulic clutches 63 and64.

The exit of the hydraulic clutch relief oil passage 223 is provided in alower portion of the second actuator body 200. The oil temperaturesensor 222 is mounted to the actuator cover 220, with its detectingportion 222 a disposed in the vicinity of the exit of the hydraulicclutch relief oil passage 223. Therefore, the temperature of the oildischarged from the first and second hydraulic clutches 63 and 64immediately after starting can be detected swiftly. This ensuresenhanced controllability of the first and second hydraulic clutches 63and 64 at the time of warm-up.

The oil return passage 225 through which the oil in the oil chamber 221is returned to the crankcase 52 side is provided in an upper portion ofthe clutch cover 56 so as to be disposed above the clutch actuator 125.Therefore, the clutch actuator 125 is located below the oil surfaceinside the oil chamber 221, and can therefore be lagged with thesurrounding oil. Consequently, variations in the temperature of theclutch actuator 125 can be reduced, and the performance of control ofthe first and second hydraulic clutches 63 and 64 can be enhanced.

Although the present invention has been described herein with respect toa number of specific illustrative embodiments, the foregoing descriptionis intended to illustrate, rather than to limit the invention. Thoseskilled in the art will realize that many modifications of theillustrative embodiment could be made which would be operable. All suchmodifications, which are within the scope of the claims, are intended tobe within the scope and spirit of the present invention.

DESCRIPTION OF REFERENCE SIGNS

-   39 . . . Crankshaft-   52 . . . Crankcase-   56 . . . Clutch cover-   60 . . . Transmission-   63 . . . First hydraulic clutch-   64 . . . Second hydraulic clutch-   69 . . . First main shaft as input shaft-   70 . . . Second main shaft as input shaft-   125 . . . Clutch actuator-   200 . . . Second actuator body-   220 . . . Actuator cover-   221 . . . Oil chamber-   222 . . . Oil temperature sensor-   222 a . . . Detecting portion-   223 . . . Hydraulic clutch relief oil passage-   224 . . . Clutch actuator relief oil passage-   225 . . . Oil return passage-   P . . . Power unit

What is claimed is:
 1. A power unit comprising: a transmission adaptedto transmit rotational power from a crankshaft, rotatably supported by acrankcase, to a drive axle with speed change, the transmissionaccommodated in the crankcase and having an input shaft rotatablysupported on the crankcase, and a hydraulic clutch which is configuredto be selectively engaged and disengaged to effect and interrupttransmission of power between the crankshaft and the input shaft, thehydraulic clutch disposed coaxially with the input shaft, wherein: aclutch cover covering the hydraulic clutch is connected to thecrankcase, a clutch actuator, which is adapted to control switching ofengaging and disengaging operations of the hydraulic clutch, and anactuator cover which covers the clutch actuator in a liquid-tight mannerare mounted to an outer surface of the clutch cover; and an oiltemperature sensor having a detecting portion for detecting atemperature of an oil accumulated in an oil chamber formed between theclutch cover and the actuator cover, is mounted to the actuator cover.2. The power unit according to claim 1, wherein a hydraulic clutchrelief oil passage, through which the oil from the hydraulic clutch isdischarged into the oil chamber, is provided in an actuator body whichconstitutes part of the clutch actuator and which is mounted to theclutch cover.
 3. The power unit according to claim 2, wherein a clutchactuator relief oil passage through which the oil discharged from theclutch actuator is discharged into the oil chamber is provided in theactuator body.
 4. The power unit according to claim 3, wherein an exitof the hydraulic clutch relief oil passage is provided at a lowerportion of the actuator body; and wherein the oil temperature sensor ismounted to the actuator cover, with the detecting portion of the oiltemperature sensor disposed in a vicinity of the exit of the hydraulicclutch relief oil passage.
 5. The power unit according to claim 4,wherein an oil return passage through which the oil in the oil chamberis returned to a crankcase side is provided in an upper portion of theclutch cover so as to be disposed above the clutch actuator.
 6. Thepower unit according to claim 3, wherein an oil return passage throughwhich the oil in the oil chamber is returned to a crankcase side isprovided in an upper portion of the clutch cover so as to be disposedabove the clutch actuator.
 7. The power unit according to claim 2,wherein an exit of the hydraulic clutch relief oil passage is providedat a lower portion of the actuator body; and wherein the oil temperaturesensor is mounted to the actuator cover, with the detecting portion ofthe oil temperature sensor disposed in a vicinity of the exit of thehydraulic clutch relief oil passage.
 8. The power unit according toclaim 7, wherein an oil return passage through which the oil in the oilchamber is returned to a crankcase side is provided in an upper portionof the clutch cover so as to be disposed above the clutch actuator. 9.The power unit according to claim 2, wherein an oil return passagethrough which the oil in the oil chamber is returned to a crankcase sideis provided in an upper portion of the clutch cover so as to be disposedabove the clutch actuator.
 10. The power unit according to claim 1,wherein an oil return passage through which the oil in the oil chamberis returned to a crankcase side is provided in an upper portion of theclutch cover so as to be disposed above the clutch actuator.
 11. Avehicle comprising a body frame and a power unit, operatively attachedto the body frame and comprising: an engine having a crankshaftrotatably supported on a crankcase, a transmission adapted to transmitrotational power from the crankshaft to a drive axle with speed change,the transmission accommodated in the crankcase and having an input shaftrotatably supported on the crankcase, and a hydraulic clutch which isconfigured to be selectively engaged and disengaged to effect andinterrupt transmission of power between the crankshaft and the inputshaft, the hydraulic clutch disposed coaxially with the input shaft,wherein: a clutch cover covering the hydraulic clutch is connected tothe crankcase, a clutch actuator, which is adapted to control switchingof engaging and disengaging operations of the hydraulic clutch, and anactuator cover which covers the clutch actuator in a liquid-tight mannerare mounted to an outer surface of the clutch cover; and an oiltemperature sensor having a detecting portion for detecting atemperature of an oil accumulated in an oil chamber formed between theclutch cover and the actuator cover, is mounted to the actuator cover.12. The vehicle according to claim 11, wherein a hydraulic clutch reliefoil passage, through which the oil from the hydraulic clutch isdischarged into the oil chamber, is provided in an actuator body whichconstitutes part of the clutch actuator and which is mounted to theclutch cover.
 13. The vehicle according to claim 12, wherein a clutchactuator relief oil passage through which the oil discharged from theclutch actuator is discharged into the oil chamber is provided in theactuator body.
 14. The vehicle according to claim 13, wherein an exit ofthe hydraulic clutch relief oil passage is provided at a lower portionof the actuator body; and wherein the oil temperature sensor is mountedto the actuator cover, with the detecting portion of the oil temperaturesensor disposed in a vicinity of the exit of the hydraulic clutch reliefoil passage.
 15. The vehicle according to claim 14, wherein an oilreturn passage through which the oil in the oil chamber is returned to acrankcase side is provided in an upper portion of the clutch cover so asto be disposed above the clutch actuator.
 16. The vehicle according toclaim 13, wherein an oil return passage through which the oil in the oilchamber is returned to a crankcase side is provided in an upper portionof the clutch cover so as to be disposed above the clutch actuator. 17.The vehicle according to claim 12, wherein an exit of the hydraulicclutch relief oil passage is provided at a lower portion of the actuatorbody; and wherein the oil temperature sensor is mounted to the actuatorcover, with the detecting portion of the oil temperature sensor disposedin a vicinity of the exit of the hydraulic clutch relief oil passage.18. The vehicle according to claim 17, wherein an oil return passagethrough which the oil in the oil chamber is returned to a crankcase sideis provided in an upper portion of the clutch cover so as to be disposedabove the clutch actuator.
 19. The vehicle according to claim 12,wherein an oil return passage through which the oil in the oil chamberis returned to a crankcase side is provided in an upper portion of theclutch cover so as to be disposed above the clutch actuator.
 20. Thevehicle according to claim 11, wherein an oil return passage throughwhich the oil in the oil chamber is returned to a crankcase side isprovided in an upper portion of the clutch cover so as to be disposedabove the clutch actuator.